Almost simultaneous with the attack on Pearl Harbor, the Japanese invaded Thailand at multiple points. In the far south, it was their entryway into Malaya. Within days, the Thai government capitulated and Imperial Japanese Army (IJA) troops began moving through the country to Burma where they intended to push out the British Colonial forces and proceed on to India.
But Allied planes and submarines were taking a heavy toll on ships carrying troops and supplies from Singapore to Burma. So the IJA revived a 50 year old British plan to build a railway connecting Bangkok to Moulmein. This was a route of some 400 kilometers the middle 200 of which were through dense uncharted jungle.
The British had estimated that it would take 3-5 years to build this link. The IJA HQ mandated 18 months. It was accomplished in 16!